1. Field of the Invention
The present invention relates to a demodulator whose equalizer is adjusted so as to match the frequency characteristic of a transmission path.
2. Related Background Art
A modulator/demodulator (modem) is required in order to transmit and receive a digital signal data to and from a general public line (analog line) by converting a digital signal into an analog signal or vice versa. The outline of a modem is shown in FIGS. 5A and 5B.
In FIGS. 5A and 5B, each circuit portion surrounded by a broken line is constructed of a digital signal processor (DSP). Reference numeral 100 represents a transmission terminal, and 118 represents a reception terminal. A scrambler 101 randomizes data in order to prevent the same data from being consecutively outputted. An encoder 102 encodes a signal from the scrambler 101 in units of a tribit, dibit or the like. A wave-shaping filter (roll-off filter) 103 prevents interruption between encoded signals. A modulator 104 modulates the encoded signal.
There are various modulating methods suitable for particular data transfer speeds. Typical methods include phase modulation which changes the phase of a carrier wave, frequency modulation (FSK) which changes the frequency of a carrier wave, amplitude modulation (AM) which changes the amplitude of a carrier wave, and quadrature amplitude modulation (QAM) which changes the amplitude and phase of a carrier wave.
A modulated signal is converted into an analog signal by a D/A converter 105 whose spurious harmonics are removed by a low-pass filter 106, and transmitted to a transmission path.
On the reception side, the components of a received signal other than those within the transmission band are removed by a band-pass filter 110. The received signal is then suppressed by an AGC 111 to a signal level suitable for being processed at the reception side, converted into a digital signal by an A/D converter 112, and demodulated by a demodulator 113 to the original signal (signal before modulation). Reference numeral 114 represents an equalizer which removes distortion components of the received signal to thereby derive the original transmitted signal, the distortion components having been added at the transmission path.
An output signal of the equalizer 114 is supplied to a judgment circuit 115 which judges a code point, and demodulated by a decoder 116. The demodulated signal randomized at the scrambler 101 is supplied to a descrambler 117 to recover the original signal at the transmission terminal 100 which is then sent to a reception terminal 118.
By using the modem described above, a digital data signal can be transmitted to and received from a general public line. In this case, the equalizer 114 operates to recover the original transmitted signal by removing distortions at the transmission path. The equalizer operation will be briefly described with reference to FIGS. 6, 7A, 7B and 7C.
The frequency characteristic of a transmission path (line) 200 of FIG. 6 is shown in FIG. 7A, and the frequency characteristic of an equalizer 201 similar to the equalizer 114 is shown in FIG. 7B.
The frequency characteristics of the line 200 and equalizer 201 are reversed as shown in FIGS. 7A and 7B. The total frequency characteristic (convolution of both characteristics, simple multiplication within the used band) is flat within the used band as shown in FIG. 7C. Signal transmission is therefore possible without distortions.
The operation of the equalizer 201 is therefore to provide the reverse characteristic of the line 200. For general data communications, the reverse characteristic of the line relative to the equalizer is first formed by transmitting known data (training data) between the transmission and reception terminals, and thereafter a control (automatic equalization or adaptative equalization) is executed so as to make the equalizer characteristic follow a gentle change of the line characteristic with time.
FIG. 8 shows an example of the structure of a conventional equalizer. Generally, an equalizer is constructed of a transversal filter. Reference numeral 400 represents a delay element for delaying a reception signal R.sub.k by a predetermined time. Each delay element 400 constitutes a flip-flop of a reception data shift register SR. Reference numeral 401 represents a register for storing a tap gain C.sub.-N to C.sub.N by which a delayed reception data of the corresponding delay element is multiplied at a corresponding multiplier 402 of a multiplier group M. Registers 401 constitute a tap gain register TGR. As well known in the art, tap gains plotted at the time axis is called a unit impulse response whose Fourier transformation is the frequency characteristic of an equalizer as shown in FIG. 7B.
Reference numeral 403 represents an adder for generating a sum of respective delayed reception data each multiplied by a tap gain. Thus, an equalizer output signal Y.sub.k is given by: ##EQU1##
The equalizer 201 sequentially calculates a tap gain for each reception data by means of a Mean Square Error (MSE) method to obtain an adaptive reverse characteristic of the line, the tap gain being given by: ##EQU2## where C.sub.l.sup.r+1 is a (r+1)-th calculated tap gain a.sub.k is an estimated value of a reception data a.sub.k, an estimated value a.sub.k for the training data being a.sub.k, .alpha.is a convergence factor (generally .alpha.&lt;&lt;1), and Y.sub.k -a.sub.k an error signal (e.sub.k).
The MSE method is an algorithm for making minimum the square error signal e.sub.k.sup.2. High speed and high precision of the equalization operation are required for a modem, and it is not too much to say that the speed and precision determine the performance of the modem.
FIG. 9 shows the structure of a circuit for judging the equalization result obtained by a conventional modem.
Referring to FIG. 9, a demodulated complex signal Ri is supplied from the demodulator at the reception side (demodulator 113 in FIG. 5B). An equalizer 500 constructed as shown in FIG. 8 recovers the original transmitted data with distortions at the line being removed.
Yi=Aie.sup.j.theta.i is a polar coordinate representation of the i-th output from the equalizer 500. A multiplier 503 multiplies an output e.sup.-j.phi..sbsp.i-1 of a complex number generator 505 by an equalizer output Yi.times.Aie.sup.j.theta.i to output Zi=Yie.sup.-j.phi..sbsp.i-1 =Aie.sup.j(.theta..sbsp.i.sup.-.phi..sbsp.-1.sup.).
A judgment unit 510 judges a code point Ai nearest a reception signal point which is an output of the multiplier 503. A subtracter 511 subtracts a code point from a reception signal point to output an error signal Ei=Zi-Ai.
The error signal Ei is multiplied by an output e.sup.j.phi..sbsp.i-1 of a complex number generator 502 to obtain Eie.sup.j.phi..sbsp.i-1 which is fed back to the equalizer 500, where e.sup.j.phi..sbsp.i-1 is a phase correction amount.
Next, a phase controller indicated as enclosed by a broken line in FIG. 9 will be described. A divider 509 divides Zi by Ai to obtain approximately e.sup.j(.theta..sbsp.i.sup.-.phi..sbsp.i-1.sup.). An imaginary part extraction unit 508 outputs sin (.theta..sub.i -.phi..sub.i-1) which is approximately equal to .theta..sub.i -.phi..sub.i-1 when .theta..sub.i .apprxeq..phi..sub.i-1.
A voltage controlled oscillator VCO 506 and low-pass filter 507 constitute a phase locked loop PLL which outputs a phase value -.phi..sub.i-1 canceling an input phase error.
The complex number generator 505 outputs e.sup.-j.phi..sbsp.i-1 which is supplied to the multiplier 503, and to a complex conjugate generator 504 which is connected to a complex number generator 502 to output e.sup.j.phi..sbsp.i-1 which is supplied to the multiplier 501, to thereby cancel the phase error of the whole system.
Channel equalization is carried out in the manner as above for data communications such as facsimile communications using a public telephone line.
Next, there will be described synchro signals used for channel equalization prior to transmission/reception of facsimile image data. Synchro signals conforming with the CCITT Recommendations V29 are shown in the following table.
__________________________________________________________________________ Sum of Segments 1 Segment 1 Segment 3 Segment 4 to 4 Format of No Segment 2 Equalizer Scrambled Sum of Line Transmission Alternative Adjusting Data Synchro Signal Energy Patterns Pattern "1" Signals __________________________________________________________________________ No. of 48 128 384 48 608 Symbol Interval Approximate 20 53 160 20 253 Time (msec) __________________________________________________________________________
The function of synchro signals for each segment will be described. Segment 1 is used during a transmission preliminary period before segment 2 and following segments. No carrier is transmitted by segment 1. Segment 2 is used during a period for setting fundamental reception operation conditions such as setting AGC, deriving timings and the like, by alternately transmitting constellation patterns stipulated by the Recommendations V29. Segment 3 is used for the initial setting of an equalizer, and uses a pseudo random pattern so as to enable smooth convergence of an equalizer. Segment 4 is used during a period for establishing synchronization of a descrambler by supplying a signal obtained when segment 1 is inputted to a scrambler.
In facsimile communications, when synchro signals are transmitted and received, consecutive "0" signals are outputted during 1.5.+-.10% second, the signals being called a training check (TCF) which checks whether or not the channel characteristic has been equalized properly. The reception terminal of a modem therefore checks if there is no error, e.g., for one second within a TCF receiving period, which is used as a standard of the degree of channel equalization. If there is an error, the reception terminal considers that the channel equalization is insufficient, and sends back a training failure (FTT) to the transmission side to thereby conduct a fall-back. If there is no error, the reception terminal sends back a reception confirmation signal (CFR) to the transmission side without a fall-back, to thereafter allow transmission/reception of image data at the transmission speed same as that of the received synchro signals.
With the above-described conventional technique, the degree of line equalization is determined simply by detecting an error during the TCF receiving period. Therefore, even if the degree of line equalization is good, a fall-back will be performed if impulse noises or short time signal disconnections occur.
Furthermore, the TCF check is carried out at the reception terminal of a modem so that the circuit portion for controlling the reception terminal is forced to deal with additional load.